Engines that meet modern emissions regulations often utilize aftertreatment systems to meet those emissions requirements. Aftertreatment systems to reduce NOx emissions can include selective catalytic reduction (SCR) components, which reduce NOx emissions in the presence of a reductant. A common reductant is urea, or alternatively ammonia, both of which end up as ammonia in the SCR component.
NOx treatment of engine exhaust, especially in systems utilizing a reductant, is particularly challenging. The ratio of NOx present to the amount of reductant should be kept within a relatively narrow range. However, the output of the engine in many applications is subject to high variability.
Accordingly, some control systems have been implemented that utilize an ammonia sensor to detect the amount of ammonia present in the exhaust. Wherever a sensor or component is utilized that is a part of the emissions control for an application, it is desirable, and sometimes required, that a failure or degradation of the system or component be detected. Detecting failures or degradation of an ammonia sensor is particularly challenging, as other components in the system do not provide secondary detection of ammonia for comparison. Therefore, further technological developments are desirable in this area.